1. Field of the Invention
The present invention relates to variable cylinder valve timing systems for internal combustion engines, and in particular to apparatus for hydraulically operating an actuator that varies a phase relationship between a crankshaft and a cam shaft.
2. Description of the Related Art
Internal combustion engines have a plurality of cylinders containing pistons that are connected to drive a crankshaft. Each cylinder has two or more valves that control the flow of air into the cylinder and the flow of exhaust gases therefrom. The valves were operated by a cam shaft which is mechanically connected to be rotated by the crankshaft. Gears, chains, or belts have been used to couple the crankshaft to the cam shaft. It is important that the valves open and close at the proper times during the combustion cycle of each cylinder. Heretofore, that valve timing relationship was fixed by the mechanical coupling between the crankshaft and the cam shaft.
The fixed setting of the valve timing often was a compromise that produced the best overall operation at all engine operating speeds. However, it has been recognized that optimum engine performance can be obtained if the valve timing varies as a function of engine speed, engine load, and other factors. With the advent of computerized engine control, it became possible to determine the optimum cylinder valve timing based on current operating conditions and in response adjust that timing accordingly.
An exemplary variable cylinder timing system is shown in FIG. 1, in which an engine computer 11 determines the optimum valve timing and applied electric current to a four-way electrohydraulic valve 10 that controls the flow of pressurized oil from a pump 13 to a cam phase actuator 12. The pump 13 typically is the conventional one used to send lubricating oil through the engine. The cam phase actuator 12 couples the cam shaft 14 to a pulley 16 that is driven by a timing belt which engages another pulley on the crankshaft of the engine. Instead of a pulley, a chain sprocket, a gear, or other device may be employed to mechanically couple the cam shaft 14 to the crankshaft. A sensor 15 provides an electrical feedback signal to the engine computer 11 indicating the angular phase of the cam shaft 14.
With additional reference to FIG. 2, the cam phase actuator 12 has a rotor 20 secured to the cam shaft 14. The cam phase actuator 12 has four vanes 22 projecting outward into four chambers 25 in the timing belt pulley 16, thereby defining first and second cavities 26 and 28 in each chamber on opposite sides to the respective vane. A first port 18 in the actuator manifold 15 is connected by a first passageway 30 to the first cavities 26 and a second passageway 33 couples a second port 19 to the second cavities 28.
By selectively controlling the application of engine oil to the first and second ports 18 and 19 of the cam phase actuator 12, the angular phase relationship between the rotating pulley 16 and the cam shaft 14 can be varied to either advance or retard the cylinder valve timing. When the electrohydraulic valve 10 is energized into the center, or neutral, position, fluid from the pump 10 is fed equally into both the first and second cavities 26 and 28 in each timing pulley chamber 25. The equal pressure on both sides of the rotor vanes 22 maintains the present position of those vanes in the pulley chambers 25. The electrohydraulic valve 10 operates in the center position the majority of the time that the engine is running Note that electric current has to be applied to the electrohydraulic valve 10 to maintain this centered position.
In another position of the electrohydraulic valve 10, pressurized oil from the pump 13 is applied to the first port 18 and other oil is exhausted from the second port 19 to a reservoir 17 (e.g., the oil pan). That pressurized oil is conveyed into the first cavities 26, thereby forcing the rotor 20 clockwise with respect to the timing belt pulley 16 and advancing the valve timing. In yet another position of electrohydraulic valve 10, pressurized oil from the pump is applied to the second port 19, while oil is exhausted from the first port 18 to the reservoir 17. Now pressurized oil is being sent into the second cavities 28, thereby forcing the rotor 20 counterclockwise with respect to the timing belt pulley 16, which retards the valve timing.
References herein to directional relationships and movement, such as left and right, or clockwise and counterclockwise, refer to the relationship and movement of the components in the orientation illustrated in the drawings, which may not be the same for the components as attached to machinery. The term “directly connected” as used herein means that the associated hydraulic components are connected together by a conduit without any intervening element, such as a valve, an orifice or other device, which restricts or controls the flow of fluid beyond the inherent restriction of any conduit. As also used herein, components that are said to be “in fluid communication” are operatively connected in a manner wherein fluid flows between those components.
Operation of the cam phase actuator 12 requires significant oil pressure and flow from the engine oil pump to overcome the torque profile of the cam shaft and adjust the cam timing. In addition, the electrohydraulic valve 10 consumes electric current while placed into the center position the majority of the engine operating time. It is desirable to reduce hydraulic and electrical energy consumption and thereby improve efficiency of the cam phasing system.